Fundamental physics is beginning to limit focused ion beam (“FIB”) performance improvements for round beams. These improvements have arisen primarily from improved electrostatic lens designs and reduced working distances, as well as from the introduction of automated variable apertures. However, at this point in time these improvements are near their fundamental limits, yet the semiconductor industry and other markets require increased milling throughput and cut quality, particularly for Fab applications.
It is becoming increasingly difficult to achieve beams with sufficient current that are small and thereby sharp enough for precision milling applications using conventional round beams. In many applications such as with slice & view applications where a “slice” is milled out of the surface of a work piece followed by an exposed cross-sectional surface being imaged, for example, by a scanning electron microscope (“SEM”), other cross-sectioning applications, and rapid transmission electron microscope (“TEM”) sample preparation, besides the need for a clean, fine cut, other capabilities from the beam are required. For example, in some of these applications, significant amounts of material must be removed. Not surprisingly, it is difficult to achieve a single beam that can satisfy all of these criteria. Even if the beam is small enough to meet sharpness requirements and have adequate resolution for precise, clean cutting, excessive time is normally needed to mill away all of the material because the beam's current is usually fairly small.
Shaped beam systems have been developed that can generate geometric shapes (such as rectangles) with straight edges for making sufficiently fine edge cuts, and at the same time, their beam spot shapes are large enough for removing (or depositing) significant quantities of material. See, for example, U.S. patent application Ser. No. 09/765,806 entitled “Shaped And Low Density Focused Ion Beams” to Gerlach et al., which is hereby incorporated by reference. It teaches methods for producing a shaped (e.g., rectangular shaped) ion beam having a relatively low current density and sharp edge resolution. In addition, it teaches both the aperture imaging (projection optics) as well as the defocused emitter imaging (shadow imaging) methods for forming shaped beams. It further teaches using a straight aperture edge at or near the beam optical axis in combination with beam under-focusing to reduce chromatic and spherical aberrations across the corresponding beam edge. In addition it teaches that a chromatic limited beam with a rectangular aperture produces a beam with constant chromatic aberration across each beam edge. The strongly under-focused shaped beams as well as the projection shaped beams are particularly attractive for beam chemistry because the current density of the shaped ion beam can be made sufficiently small that the etching or deposition rate is not limited by the exhaustion of adsorbed gas molecules, and in addition, the overall beam current can be made sufficiently high to achieve satisfactory etch and deposition rates. However, such systems may not fully address the high current, high current density, and unique density shape requirements for improved milling resolution and throughput desired in many applications.
Accordingly, what is needed is a method and system for generating a shaped beam having desired current, current density, and shape characteristics for particular milling and material deposition applications.